Friday said:
1 - How essential is Robinson's Atlas arc?
Not very. It introduces Atlas but doesn't reveal much about him, and also gives a glimpse (briefly) at Project 7734 and the mystery man behind that, which is shaping up to be one of the principal villains of the story. It's kind of a fun arc, but not essential.
Friday said:
2 - is it keeping up a head of steam or is it meandering?
It's pretty meandering. There's decent quality to it all, but Robinson in particular likes to juggle a large tangential cast and sort of placidly wander towards the story.
Friday said:
3 - Are the Johns collections before it required? I'll probably get them anyway but it'd be nice to know.
not essential, but they provide background.
Last Son introduces Zod, his wife, and their son, who are all key characters in the present storyline.
Brainiac gives us a pretty cool reimagining of the character that also introduces Kandor and develops New Krypton as a plot element.
As for the books themselves, so far they haven't relied too much on crossover, but now we're going head-first into an all-books mash-up.
Supergirl has mostly worked on building an ongoing supporting cast for Kara that meshes with the rest of the Superman mythos. It's tied her into Daily Planet staff as well as provided established villains which tie back to Project 7734. It's largely about the conflict of heritage she faces: Kryptonian vs. Earth.
Action Comics has established two new heroes from New Krypton who are on a secret mission to eliminate hostile Kryptonians hidden on Earth.
Superman: World of New Krypton has largely concerned itself with detailing the peculiarities and politics of Kryptonian culture. Superman's come to New Krypton to keep an eye on Zod, and in the process, we get a look at how things work on his home world. It's probably the best book of the bunch, and provides a decent amount of twists and turns.
Superman has dealt with Metropolis in Superman's absence. We follow Guardian and the Science Police, Mon-El, Zatara, Steel, and a few others. It's the most meandering of the bunch, and as a result, there's issues that Robinson really hits out of the park, and then others that seem to be completely filler.
All in all, the books are decent but not spectacular. None of the books are bad, but they're rarely great either.